Black Phosphorus–Tungsten Oxide Sandwich-like Nanostructures for Highly Selective NO2 Detection

Modern chemical production processes often emit complex mixtures of gases, including hazardous pollutants such as NO2. Although widely used, gas sensors based on metal oxide semiconductors such as WO3 respond to a wide range of interfering gases other than NO2. Consequently, developing WO3 gas sensors with high NO2 selectivity is challenging. In this study, a simple one-step hydrothermal method was used to prepare WO3 nanorods modified with black phosphorus (BP) flakes as sensitive materials for NO2 sensing, and BP-WO3-based micro-electromechanical system gas sensors were fabricated. The characterization of the as-prepared BP-WO3 composite through X-ray diffraction scanning electron microscopy and X-ray photoelectron spectroscopy confirmed the successful formation of the sandwich-like nanostructures. The result of gas-sensing tests with 2–14 ppm NO2 indicated that the sensor response was 1.25–2.21 with response–recovery times of 36 and 36 s, respectively, at 190 °C. In contrast to pure WO3, which exhibited a response of 1.07–2.2 to 0.3–5 ppm H2S at 160 °C, BP-WO3 showed almost no response to H2S. Thus, compared with pure WO3, BP-WO3 exhibited significantly improved NO2 selectivity. Overall, the BP-WO3 composite with sandwich-like nanostructures is a promising material for developing highly selective NO2 sensors for practical applications.


Introduction
With the rapid development of modern industry, various health-threatening atmospheric pollutants have emerged.A representative example is the inorganic compound NO 2 , a toxic gas with an irritating odor [1].NO 2 inhalation can lead to interstitial lung lesions, respiratory infections, and lung fibrosis [2,3].Anthropogenic NO 2 mainly originates from high-temperature combustion processes and is released through fuel combustion, motor vehicle exhaust, and boiler exhaust emissions [4].In practice, NO 2 is often accompanied by interfering gases such as H 2 S, CO, and SO 2 .Therefore, sensors with high selectivity and response must be developed to meet NO 2 detection requirements.
In recent years, resistive gas sensors based on metal oxide semiconductors (MOS) have been widely used for NO 2 detection because of their low cost, environmental friendliness, and excellent performance.Hydrothermal methods for oxide synthesis are being widely utilized today.Jamnani et al. synthesized Sm 2 O 3 nanorods through a straightforward hydrothermal process, employing cetyltrimethylammonium bromide (CTAB) as an organic surfactant for detecting volatile organic compounds; they achieved a notable response of 3.41-1 ppm for acetone [5].Furthermore, flower-like MoS 2 /WS 2 composites were prepared via a simple one-pot hydrothermal method, enhancing the detection of NO 2 at room temperature [6].MOS materials such as WO 3 [7], SnO 2 [8], In 2 O 3 [9], ZnO [10], and CuO [11] are promising for meeting the low-limit, high-response requirements of NO 2 detection.Additionally, Ag 2 Te, an n-type semiconductor, exhibited commendable NO 2 Sensors 2024, 24, 1376 2 of 12 sensing performance.The resistance of the Ag 2 Te sensor remained stable in air and at 1 ppm NO 2 over a humidity range of 10-90%, demonstrating its excellent humidity-resistant properties [12].
WO 3 is a typical n-type MOS with an adjustable bandgap of 2.5-2.8eV [13].Owing to its crystal structure and surface properties, WO 3 can adopt various microscopic morphologies including nanowires [14], nanospheres [15], nanosheets [16], and urchin-like nanostructures [17].The microscopic morphology of WO 3 can be adjusted to strengthen the interactions with gases and increase the possibility of gas adsorption.However, similar to sensors based on other metal oxides, WO 3 nanomaterial gas sensors exhibit poor selectivity and cannot accurately detect NO 2 in complex environments in which multiple gases are mixed [18].
MOSs have been doped with different catalysts to improve sensor selectivity and increase the reaction rate between the target gas and sensing layer.Based on electronic sensitization and chemical sensitization mechanisms [19,20], doping with an appropriate amount of a noble metal can improve the gas response of MOS materials.For example, compared with pure ZnO, Pd-modified ZnO nanowires improved response and significantly higher NO 2 selectivity [21].As an alternative approach, gas sensors have been prepared using composites of MOSs and reduced graphene oxide (rGO), which has a large surface area and exhibits high electron mobility and high conductivity at low temperatures [22].Notably, SnO 2 nanofibers sensitized with rGO nanosheets achieved high response and selectivity for acetone and H 2 S [23].In addition, recently, phosphorene has garnered research interest owing to its honeycomb structure, high carrier mobility, and tunable bandgap, which ranges from 0.3 eV in bulk to approximately 1.9 eV in monolayers [24].Black phosphorus (BP), a novel single-element two-dimensional layered material, has been extensively utilized in gas sensing [25].Han fabricated a BP gas sensor featuring high-quality, few-layered BP micro-ribbons.Energy-dispersive spectroscopy mapping revealed that BP underwent only slight oxidation, achieving low-limit detection and high NO 2 selectivity under both N 2 and air conditions [26].Zhou fabricated a two-dimensional SnO 2 nanosheet micro-electromechanical system (MEMS) gas sensor decorated with black phosphorus (BP) to detect H 2 S [27].The BP-SnO 2 sensor had a lower optimal operating temperature and faster response-recovery times and exhibited higher selectivity than the pure SnO 2 sensor.When BP, which is a p-type semiconductor, is added to WO 3 , which is an n-type semiconductor, more p-n heterojunctions can be formed.Moreover, the excellent conductivity of BP can increase the carrier mobility of WO 3 .Therefore, BP-WO 3 composites are of interest for gas-sensing applications.
We fabricated a highly selective BP-WO 3 gas sensor for NO 2 detection in this study.BP crystals were prepared via the chemical vapor-phase transport method, and BP-WO 3 sandwich-like nanostructures were formed by a simple one-step hydrothermal method.The successful preparation of the BP-WO 3 composite was confirmed through its characterization via scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS).Subsequently, a gas-sensing test platform was constructed and the selectivity, response, operating temperature, and concentration range of the BP-WO 3 sensor were demonstrated to be superior to those of pure WO 3 .This improved sensing performance could be attributed to the enhanced electron-trapping ability of BP-WO 3 .

Preparation of Sensing Materials
BP crystals were prepared via the chemical vapor-phase transport method.First, 0.5 g of red phosphorus, 0.07 g of Sn powder, and 0.03 g of I 2 were placed in a quartz glass tube with an outer diameter of 18 mm.The quartz tube was then pumped to a vacuum and sealed by fusing with a cylindrical quartz block.The end of the quartz tube containing the raw materials (hot end) was placed in the center of the high-temperature zone of a tube furnace.The heating stage was heated to 600 • C at a rate of 20 • C/h and held for 2 h, cooled to 500 • C at a rate of 10 • C/h and held for 2 h, and finally cooled to 25 • C at a rate of 15 • C/h.The BP crystals formed at the cold end of the quartz tube.The BP crystals were then ground into a fine powder and a 10 mg/mL aqueous solution was prepared.
WO 3 was prepared via a hydrothermal method.First, 2.05 g of Na 2 WO 4 •2H 2 O, 0.25 g of Na 2 SO 4 , and 0.3 g of K 2 SO 4 were added to 40 mL of deionized water.After stirring for 20 min, the pH was adjusted to 1.6 with 2 M HCl.The precursor solution was transferred to a 100 mL Teflon-lined autoclave and heated at 180 • C for 24 h.After cooling to 25 • C, the precipitate was centrifuged at 4000 rpm for 15 min and washed with water and anhydrous ethanol (6 times).Finally, the WO 3 sample was obtained by vacuum drying at 60 • C for 8 h.The WO 3 synthesis process is outlined in Figure 1.The BP-WO 3 composite was synthesized using the same method, except that 1 mL of the aqueous BP solution was added after adjusting the pH.
Sensors 2024, 24, x FOR PEER REVIEW 3 of 14 the raw materials (hot end) was placed in the center of the high-temperature zone of a tube furnace.The heating stage was heated to 600 °C at a rate of 20 °C/h and held for 2 h, cooled to 500 °C at a rate of 10 °C/h and held for 2 h, and finally cooled to 25 °C at a rate of 15 °C/h.The BP crystals formed at the cold end of the quartz tube.The BP crystals were then ground into a fine powder and a 10 mg/mL aqueous solution was prepared.WO3 was prepared via a hydrothermal method.First, 2.05 g of Na2WO4•2H2O, 0.25 g of Na2SO4, and 0.3 g of K2SO4 were added to 40 mL of deionized water.After stirring for 20 min, the pH was adjusted to 1.6 with 2 M HCl.The precursor solution was transferred to a 100 mL Teflon-lined autoclave and heated at 180 °C for 24 h.After cooling to 25 °C, the precipitate was centrifuged at 4000 rpm for 15 min and washed with water and anhydrous ethanol (6 times).Finally, the WO3 sample was obtained by vacuum drying at 60 °C for 8 h.The WO3 synthesis process is outlined in Figure 1.The BP-WO3 composite was synthesized using the same method, except that 1 mL of the aqueous BP solution was added after adjusting the pH.

Characterization of Sensing Materials
The crystal structures of WO3 and BP-WO3 were analyzed using an X-ray diffractometer (SmartLab 3 kW, Rigaku Corporation, Tokyo, Japan) using Cu Kα radiation in the scanning range of 20°-70°.The valence states of the elements were analyzed using an Xray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA).The morphologies of the samples were observed through field-emission SEM (ZEISS Gemini SEM 300, Carl Zeiss AG, Oberkochen, Germany) at acceleration voltages of 3-15 kV.

Test Platform for Gas Sensing
Sensors were fabricated by coating the sensing material on a MEMS substrate, consisting of an interdigitated electrode, an insulating layer, a heating electrode, a supporting layer, and a silicon substrate (Figure 2a,b).Specifically, the sensing material was ground in an agate mortar to 200 mesh and sieved.A slurry was prepared by uniformly mixing this sample with terpineol.The slurry was coated on the sensing layer of the MEMS substrate, which was then vacuum dried at 65 °C for 5 h and aged at the operating temperature for 1 week.Figure 2c,d show the physical images of the sensor.The test platform comprised a 7.5 L closed air chamber, fan, DC power supply, data-acquisition unit, and PC (Figure 2e).A syringe was used to inject a specific volume of gas into the gas chamber.The fan diffused the gas evenly and quickly into the gas chamber, and the DC power

Characterization of Sensing Materials
The crystal structures of WO 3 and BP-WO 3 were analyzed using an X-ray diffractometer (SmartLab 3 kW, Rigaku Corporation, Tokyo, Japan) using Cu Kα radiation in the scanning range of 20 • -70 • .The valence states of the elements were analyzed using an X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA).The morphologies of the samples were observed through field-emission SEM (ZEISS Gemini SEM 300, Carl Zeiss AG, Oberkochen, Germany) at acceleration voltages of 3-15 kV.

Test Platform for Gas Sensing
Sensors were fabricated by coating the sensing material on a MEMS substrate, consisting of an interdigitated electrode, an insulating layer, a heating electrode, a supporting layer, and a silicon substrate (Figure 2a,b).Specifically, the sensing material was ground in an agate mortar to 200 mesh and sieved.A slurry was prepared by uniformly mixing this sample with terpineol.The slurry was coated on the sensing layer of the MEMS substrate, which was then vacuum dried at 65 • C for 5 h and aged at the operating temperature for 1 week.Figure 2c,d show the physical images of the sensor.The test platform comprised a 7.5 L closed air chamber, fan, DC power supply, data-acquisition unit, and PC (Figure 2e).A syringe was used to inject a specific volume of gas into the gas chamber.The fan diffused the gas evenly and quickly into the gas chamber, and the DC power supply provided the working voltage of the sensor.Gas adsorption onto the gas-sensitive material caused a change in the sensor resistance, which was recorded by the data-acquisition unit and then uploaded to the PC.
supply provided the working voltage of the sensor.Gas adsorption onto the gas-sensitive material caused a change in the sensor resistance, which was recorded by the data-acquisition unit and then uploaded to the PC.112), (202), and (220) crystal planes, respectively.However, because of the extremely low BP content of BP-WO 3 , the XRD pattern of this sample does not contain any diffraction peaks corresponding to phosphorus.Therefore, the samples were further characterized using other methods.

Material Characterization
this sample does not contain any diffraction peaks corresponding to phosphorus.Therefore, the samples were further characterized using other methods.Figure 4a-c show the SEM images of pure WO3.The microscopic morphology of pure WO3 consists of rod-like structures with diameters of 50 nm and lengths ranging from 100 nm to 1 μm, resulting in a rectangular cross-section.Most nanorods are stacked in parallel rather than dispersed, indicating the presence of strong interaction forces between them.Remarkably, the rod-like structures consist of many fine nanorods (5-10 nm in diameter) assembled into bundles.Energy-dispersive X-ray spectroscopy (EDS) results extracted from the region in Figure 4b show that W and O were uniformly distributed on the nanorods (Figure 4d,e).Based on the characteristic W and O peaks (Figure 4f), the mass fractions of W and O were calculated to be 80.95% and 19.05%, respectively.
Figure 4g-i show the SEM images of BP-WO3.The BP-WO3 composite was found to possess a novel sandwich-like nanostructure assembled from WO3 nanorods and multilayer BP nanosheets.As shown in Figure 4g, the BP nanosheets provide attachment sites for nanowire growth, and both sides of each BP nanosheet are uniformly and densely loaded with nanorods.Notably, the surrounding free WO3 nanorods and BP nanosheets do not bind together.According to a previous study, monolayer BP nanosheets have a thickness of 3-5 nm after ultrasonic liquid-phase stripping.In the BP-WO3 composite, the nanosheet in the middle position has a thickness of 27.66 nm (Figure 4h), which suggests that this structure consists of 5-9 layers of monolayer BP nanosheets.The magnified SEM images in Figure 4h,j reveal nearly perpendicular nanorod growth on both sides of the BP nanosheets.Moreover, these nanorods are of the same length and cover the nanosheets almost completely.This structure explains why no P was detected through XRD or EDS. Figure 4a-c show the SEM images of pure WO 3 .The microscopic morphology of pure WO 3 consists of rod-like structures with diameters of 50 nm and lengths ranging from 100 nm to 1 µm, resulting in a rectangular cross-section.Most nanorods are stacked in parallel rather than dispersed, indicating the presence of strong interaction forces between them.Remarkably, the rod-like structures consist of many fine nanorods (5-10 nm in diameter) assembled into bundles.Energy-dispersive X-ray spectroscopy (EDS) results extracted from the region in Figure 4b show that W and O were uniformly distributed on the nanorods (Figure 4d,e).Based on the characteristic W and O peaks (Figure 4f), the mass fractions of W and O were calculated to be 80.95% and 19.05%, respectively.XPS was performed to further investigate the chemical binding states of the samples.Figure 5a shows the XPS profiles of pure WO3 and BP-WO3.In the WO3 sample, the atomic ratio of W to O is 24.02%:75.98%,which corresponds well with the expected atomic ratio (1:3).However, because the XPS beam can only penetrate the sample to a depth of 3-10 nm, the analysis is limited to the sample surface and P was not detected.Figure 5b and c show the W 4f and O 1s XPS spectra of the BP-WO3 sample.The W 4f spectrum exhibits two major peaks centered at 37.9 and 35.7 eV [28], corresponding to W 6+ 4f5/2 and 4f7/2, respectively.The O 1s spectrum exhibits a major peak at 530.5 eV [29], corresponding to the  for nanowire growth, and both sides of each BP nanosheet are uniformly and densely loaded with nanorods.Notably, the surrounding free WO 3 nanorods and BP nanosheets do not bind together.According to a previous study, monolayer BP nanosheets have a thickness of 3-5 nm after ultrasonic liquid-phase stripping.In the BP-WO 3 composite, the nanosheet in the middle position has a thickness of 27.66 nm (Figure 4h), which suggests that this structure consists of 5-9 layers of monolayer BP nanosheets.The magnified SEM images in Figure 4h,j reveal nearly perpendicular nanorod growth on both sides of the BP nanosheets.Moreover, these nanorods are of the same length and cover the nanosheets almost completely.This structure explains why no P was detected through XRD or EDS.
XPS was performed to further investigate the chemical binding states of the samples.Figure 5a shows the XPS profiles of pure WO 3 and BP-WO 3 .In the WO 3 sample, the atomic ratio of W to O is 24.02%:75.98%,which corresponds well with the expected atomic ratio (1:3).However, because the XPS beam can only penetrate the sample to a depth of 3-10 nm, the analysis is limited to the sample surface and P was not detected.Figure 5b and c show the W 4f and O 1s XPS spectra of the BP-WO 3 sample.The W 4f spectrum exhibits two major peaks centered at 37.9 and 35.7 eV [28], corresponding to W 6+ 4f 5/2 and 4f 7/2 , respectively.The O 1s spectrum exhibits a major peak at 530.5 eV [29], corresponding to the lattice oxygen (O 2− ) of WO 3 , and a small peak at 532.1 eV [30], ascribed to the chemisorbed oxygen or weakly bonded oxygen species on the WO 3 nanorods.

Gas-Sensing Properties
The operating temperature is a crucial sensor metric for actual measurement.To explore the effect of the operating temperature on the sensor performance, we performed static measurements using the BP-WO3 sensor at 10 ppm NO2. Figure 6a shows the variation in the resistance of the BP-WO3 sensor over time at temperatures between 190 and 270 °C.The initial resistance of the sensor reaches tens of megahertz and decreases with increasing temperature.The resistance increases significantly after the target gas is introduced and slowly returns to the initial level after releasing the gas.The lowest operating temperature point was selected at 190 °C to avoid large measurement errors caused by high sensor resistance.Figure 6b illustrates the response (Ra/Rg, where Ra is the resistance of the sensor in air and Rg is the resistance of the sensor in the target gas) at different temperatures.Increasing the temperature induces a significant decrease in response; however, the response-recovery times of the sensor become faster.The response of the sensor is only 1.14 at 270 °C but increases to 1.73 at 190 °C.The response-recovery times (defined as the time required for a change of the resistance of 90%) of the sensor are 72 and 92 s,

Gas-Sensing Properties
The operating temperature is a crucial sensor metric for actual measurement.To explore the effect of the operating temperature on the sensor performance, we performed static measurements using the BP-WO 3 sensor at 10 ppm NO 2 .Figure 6a shows the variation in the resistance of the BP-WO 3 sensor over time at temperatures between 190 and 270 • C. The initial resistance of the sensor reaches tens of megahertz and decreases with increasing temperature.The resistance increases significantly after the target gas is introduced and slowly returns to the initial level after releasing the gas.The lowest operating temperature point was selected at 190 • C to avoid large measurement errors caused by high sensor resistance.Figure 6b illustrates the response (R a /R g , where R a is the resistance of the sensor in air and R g is the resistance of the sensor in the target gas) at different temperatures.Increasing the temperature induces a significant decrease in response; however, the response-recovery times of the sensor become faster.The response of the sensor is only 1.14 at 270 To verify the NO2 selectivity of the sensor, we performed gas sensitivity tests with the BP-WO3 and pure WO3 sensors using H2S as a representative interfering gas. Figure 7a shows the response (Ra/Rg) of the pure WO3 sensor to 4 ppm H2S at different operating temperatures.The sensor response decreases drastically with increasing temperature, but the response-recovery times become significantly shorter.Although the response is higher at 160 °C than at 240 °C, the response-recovery times decrease significantly (from 60 and 100 s, respectively, at 160 °C, to 13 and 15 s, respectively, at 240 °C), consistent with the trend observed for BP-WO3.The investigation of the response of the pure WO3 sensor to H2S at the concentration of 0.3-5 ppm (Figure 7b) reveals good resolving power in the range of 1-5 ppm.However, the difference in response is negligible at concentrations of 0.3 and 0.5 ppm (1.024 and 1.064, respectively), suggesting that the detection limit has been reached.Figure 7c shows the relationship between the response of the pure WO3 sensor and H2S concentration in the range of 0.3-5 ppm.The results show a good linear relationship (R 2 = 0.97) with an equation of y = 0.9617 + 0.2225x, indicating the good calibratable capability of the pure WO3 sensor.Based on the linear fit results, the theoretical pure WO3 response to 10 ppb H2S can be calculated as 0.96.In contrast, the BP-WO3 sensor exhibited no response to various H2S concentrations (Figure 7d), demonstrating that the BP-WO3 sensor can effectively avoid the influence of the interfering gas, H2S, unlike pure WO3, resulting in improved selectivity.Figure 6c shows the response of the BP-WO 3 sensor to NO 2 concentrations in the range of 2-14 ppm.The sensor response decreases significantly as the gas concentration decreases, indicating an excellent resolving power.In addition, the sensor exhibits fast response-recovery times for NO 2 .Figure 6d shows the relationship between the response of the BP-WO 3 sensor and NO 2 concentration in the 2-14 ppm range.The results show an excellent linear relationship (R 2 = 0.98) with an equation of y = 1.1522 + 0.0613x, implying that the BP-WO 3 sensor has good calibration capability.Based on the linear fit results, the theoretical BP-WO 3 response to 10 ppb NO 2 can be calculated as 1.15.
To verify the NO 2 selectivity of the sensor, we performed gas sensitivity tests with the BP-WO 3 and pure WO 3 sensors using H 2 S as a representative interfering gas. Figure 7a shows the response (R a /R g ) of the pure WO 3 sensor to 4 ppm H 2 S at different operating temperatures.The sensor response decreases drastically with increasing temperature, but the response-recovery times become significantly shorter.Although the response is higher at 160 • C than at 240 • C, the response-recovery times decrease significantly (from 60 and 100 s, respectively, at 160 • C, to 13 and 15 s, respectively, at 240 • C), consistent with the trend observed for BP-WO 3 .The investigation of the response of the pure WO 3 sensor to H 2 S at the concentration of 0.3-5 ppm (Figure 7b) reveals good resolving power in the range of 1-5 ppm.However, the difference in response is negligible at concentrations of 0.3 and 0.5 ppm (1.024 and 1.064, respectively), suggesting that the detection limit has been reached.Figure 7c shows the relationship between the response of the pure WO 3 sensor and H 2 S concentration in the range of 0.3-5 ppm.The results show a good linear relationship (R 2 = 0.97) with an equation of y = 0.9617 + 0.2225x, indicating the good calibratable capability of the pure WO 3 sensor.Based on the linear fit results, the theoretical pure WO 3 response to 10 ppb H 2 S can be calculated as 0.96.In contrast, the BP-WO 3 sensor exhibited no response to various H 2 S concentrations (Figure 7d), demonstrating that the BP-WO 3 sensor can effectively avoid the influence of the interfering gas, H 2 S, unlike pure WO 3 , resulting in improved selectivity.Figure 8a,b show the response curves of BP-WO3 and pure WO3 over seven cycles at 12 ppm NO2 and 3.5 ppm H2S, respectively.The response values and response-recovery times of the sensor during each cycle are similar, indicating that the BP-WO3 and pure WO3 sensors have excellent repeatability.In addition, we tested the responses of the BP-WO3 and pure WO3 sensors to 1 ppm H2S, NO2, C2H6O, CO2, and CO (Figure 8b).The pure WO3 sensor responded well to both H2S and NO2, whereas the BP-WO3 sensor exhibited almost no response to H2S, confirming that the BP-WO3 sensor exhibits better NO2 selectivity than the pure WO3 sensor.The response values of the BP-WO3 sensor to NO2 and H2S over 15 days are shown in Figure 8c.Over this period, the response of the sensor to NO2 only decreases slightly, with the retention of more than 95% of the initial performance.Moreover, the sensor remains unresponsive to H2S for 15 days, demonstrating the long-term stability of BP-WO3.In addition, the gas-sensing properties of previously reported WO3-based sensors and the as-prepared system are summarized in Table 1.It can be seen that the BP-WO3 sensor demonstrates similar performance to or outperforms other sensors.8b).The pure WO 3 sensor responded well to both H 2 S and NO 2 , whereas the BP-WO 3 sensor exhibited almost no response to H 2 S, confirming that the BP-WO 3 sensor exhibits better NO 2 selectivity than the pure WO 3 sensor.The response values of the BP-WO 3 sensor to NO 2 and H 2 S over 15 days are shown in Figure 8c.Over this period, the response of the sensor to NO 2 only decreases slightly, with the retention of more than 95% of the initial performance.Moreover, the sensor remains unresponsive to H 2 S for 15 days, demonstrating the long-term stability of BP-WO 3 .In addition, the gas-sensing properties of previously reported WO 3 -based sensors and the as-prepared system are summarized in Table 1.It can be seen that the BP-WO 3 sensor demonstrates similar performance to or outperforms other sensors.

Gas-Sensing Mechanism
The gas-sensing mechanism of WO3 is similar to that of other MOSs, where charge transfer to the gas during adsorption leads to a change in resistance.When the sensor is exposed to air, oxygen molecules adsorb on the surface of the WO3 nanorods and capture free electrons from the WO3 conduction band.Thus, the oxygen molecules are converted to the chemisorbed oxygen species (O and O ) [37], which form an electron-withdrawing layer near the surface of the WO3 nanorods.Consequently, the carrier concentration

Gas-Sensing Mechanism
The gas-sensing mechanism of WO 3 is similar to that of other MOSs, where charge transfer to the gas during adsorption leads to a change in resistance.When the sensor is exposed to air, oxygen molecules adsorb on the surface of the WO 3 nanorods and capture free electrons from the WO 3 conduction band.Thus, the oxygen molecules are converted to the chemisorbed oxygen species (O − 2 and O − ) [37], which form an electron-withdrawing layer near the surface of the WO 3 nanorods.Consequently, the carrier concentration of WO 3 , which is an n-type semiconductor, decreases, leading to an increase in resistance.Upon introducing NO 2 , NO 2 molecules adsorb on the WO 3 surface and react with the adsorbed oxygen ions [38].This process can be described by Equations ( 1)-(5).
O 2 (gas) → O 2 (ads), O 2 (ads) + e − → O − 2 (ads), O − 2 (ads) + e − → 2O − (ads), repeatability.The high selectivity of BP-WO 3 was attributed to the surface oxygen ions having a much higher reaction rate with NO 2 with than with the interfering gases.The use of the as-prepared BP-WO 3 composite will allow for the enhancement of the selectivity of gas sensors.

Figure 1 .
Figure 1.Schematic of the preparation of WO3 nanorods.

Figure 1 .
Figure 1.Schematic of the preparation of WO 3 nanorods.

Figure 2 .
Figure 2. (a,b) Schematics of micro-electromechanical system (MEMS) structure; images of the MEMS substrate (c) before and (d) after coating with the sensing material; (e) test platform for gas sensing.

Figure 2 .
Figure 2. (a,b) Schematics of micro-electromechanical system (MEMS) structure; images of the MEMS substrate (c) before and (d) after coating with the sensing material; (e) test platform for gas sensing.

Figure 4 .
Figure 4. (a-c) SEM images of pure WO3; EDS elemental mapping profiles of (d) W and (e) O in pure WO3; (f) EDS spectrum of pure WO3; (g-i) SEM images of BP-WO3.

Figure 4 .
Figure 4. (a-c) SEM images of pure WO 3 ; EDS elemental mapping profiles of (d) W and (e) O in pure WO 3 ; (f) EDS spectrum of pure WO 3 ; (g-i) SEM images of BP-WO 3 .

Figure
Figure 4g-i show the SEM images of BP-WO 3 .The BP-WO 3 composite was found to possess a novel sandwich-like nanostructure assembled from WO 3 nanorods and multilayer BP nanosheets.As shown in Figure 4g, the BP nanosheets provide attachment sites

14 Figure 6 .
Figure 6.(a) Resistance and (b) response of the BP-WO3 sensor to 10 ppm NO2 at various operating temperatures; (c) response of the BP-WO3 sensor to various NO2 concentrations at 190 °C; (d) linear fitting of the BP-WO3 sensor response as a function of NO2 concentration.

Figure 6 .
Figure 6.(a) Resistance and (b) response of the BP-WO 3 sensor to 10 ppm NO 2 at various operating temperatures; (c) response of the BP-WO 3 sensor to various NO 2 concentrations at 190 • C; (d) linear fitting of the BP-WO 3 sensor response as a function of NO 2 concentration.

Sensors 2024 , 14 Figure 7 .
Figure 7. (a) Response of the WO3 sensor to 4 ppm H2S at various operating temperatures; (b) response of WO3 sensor to various H2S concentrations at 160 °C; (c) linear fitting of the WO3 sensor response as a function of H2S concentration; (d) response of the WO3 and BP-WO3 sensors as a function of H2S concentration.

Figure 7 .
Figure 7. (a) Response of the WO 3 sensor to 4 ppm H 2 S at various operating temperatures; (b) response of WO 3 sensor to various H 2 S concentrations at 160 • C; (c) linear fitting of the WO 3 sensor response as a function of H 2 S concentration; (d) response of the WO 3 and BP-WO 3 sensors as a function of H 2 S concentration.

Figure
Figure8a,b show the response curves of BP-WO 3 and pure WO 3 over seven cycles at 12 ppm NO 2 and 3.5 ppm H 2 S, respectively.The response values and response-recovery times of the sensor during each cycle are similar, indicating that the BP-WO 3 and pure WO 3 sensors have excellent repeatability.In addition, we tested the responses of the BP-WO 3 and pure WO 3 sensors to 1 ppm H 2 S, NO 2 , C 2 H 6 O, CO 2 , and CO (Figure8b).The pure WO 3 sensor responded well to both H 2 S and NO 2 , whereas the BP-WO 3 sensor exhibited almost no response to H 2 S, confirming that the BP-WO 3 sensor exhibits better NO 2 selectivity than the pure WO 3 sensor.The response values of the BP-WO 3 sensor to NO 2 and H 2 S over 15 days are shown in Figure8c.Over this period, the response of the sensor to NO 2 only decreases slightly, with the retention of more than 95% of the initial performance.Moreover, the sensor remains unresponsive to H 2 S for 15 days, demonstrating the long-term stability of BP-WO 3 .In addition, the gas-sensing properties of previously reported WO 3 -based sensors and the as-prepared system are summarized in Table1.It can be seen that the BP-WO 3 sensor demonstrates similar performance to or outperforms other sensors.

Figure 8 .
Figure 8.(a) Repeatability of the BP-WO3 sensor response to 12 ppm NO2; (b) repeatability of the WO3 sensor response to 3.5 ppm H2S; (c) selectivities of the BP-WO3 and WO3 sensors for various gases; (d) response of the BP-WO3 sensor to 12 ppm NO2 and 4 ppm H2S over 15 days.

Figure 8 .
Figure 8.(a) Repeatability of the BP-WO 3 sensor response to 12 ppm NO 2 ; (b) repeatability of the WO 3 sensor response to 3.5 ppm H 2 S; (c) selectivities of the BP-WO 3 and WO 3 sensors for various gases; (d) response of the BP-WO 3 sensor to 12 ppm NO 2 and 4 ppm H 2 S over 15 days.

Table 1 .
Gas-sensing properties of various WO 3 -based NO 2 gas sensors.